Does my data get uploaded anywhere?
No. Everything runs locally in your browser and nothing is sent to a server.
Is this free to use?
Yes. It is completely free, with no account, no signup, and no usage limits.
Does it work on a phone?
Yes. The page runs in any modern mobile browser, on iPhone and Android alike, with no app to install.
Why does the output have no leading zeros?
Because a binary number, like a decimal one, has no inherent width. 5 is 101, and whether you write it as 00000101 depends on whether you are storing it in a byte. Pad the output to 8, 16 or 32 characters yourself if your context needs a fixed width.
How do I write a negative number in binary?
This tool prints a minus sign and the magnitude, so -10 becomes -1010. Computers instead use two's complement at a fixed width: as a signed 8-bit byte, -10 is 11110110. To get that, take the positive binary value, invert every bit, and add one.
How do I convert decimal to binary by hand?
Divide by 2 repeatedly and keep the remainders. For 13: 13/2 is 6 remainder 1, 6/2 is 3 remainder 0, 3/2 is 1 remainder 1, 1/2 is 0 remainder 1. Reading the remainders backwards gives 1101.
Can it handle very large numbers?
Yes. The conversion runs on arbitrary-precision integers, so numbers well beyond the 9,007,199,254,740,991 limit of a double convert exactly. A spreadsheet doing the same job would start rounding.
What is 255 in binary?
11111111, eight ones, which is the largest value a single byte can hold. 256 rolls over into 100000000, nine bits, which is exactly why an 8-bit counter wraps back to zero after 255.